The really good stuff, such as Cardini's Caesar and Ranch dressings and the late Paul Newman's range, have vanished from the shops during recent years, not even appearing for the summer season let alone all year round.
At least as far as oil-based dressings are concerned, there is Maille's vinaigrette with "a touch of black olives" (as it says on the label), and this I have found to be very good with none of the sickliness of the others on the market – whether a supermarket's own brand, Kraft, Hellman's or any other I have found for sale.
I still miss the Newman's and Cardini dressings, especially the latter as the two I mentioned are creamy dressings which have their particular uses and for which I have no current alternative, but at least I now have something usable.
An excellent creamy dressing that one cannot buy separately is the chive-and-herb dressing that comes with Sainsbury's "classic" tray salad, in a sachet. Now, if only that were available in a bottle...